Wind build up speed with sapling addon
When creating your animated tree, note all values and use the export preset. (It doesn't export all settings, that's why we have to manually input the rest.)


Go to the dope sheet and choose the action editor mode. You will see that a windAction has been created. Press the F-button next to the action's name to give it a fake user. Delete the tree. Since you gave the action the fake user, it is still existent, even though the armature is deleted.


Create the same tree with the same generation values (which we wrote down earlier). Change the animation settings as desired e.g for a stronger wind.


A new action is created next to the existent one. I renamed it strongerWind.


Go to the NLA editor and add a track with ShiftA. Add an action with ShiftA as well.


Check sync length and extend the action by increasing the end frame. Add the other action as well.


Offset the NLA strips. By scrubbing over the frames you will see, that either action is played in the 3D view. Select the first action and enable Animated Influence. Set two keyframes on the influence with the I key. Select the 2nd action and enable auto blend. Auto blend creates a blend between two overlapping action strips.


Set the first action to additive blending. Go to the graph editor, you will see the two keyframes for the animated influence, which we created. Here you can add more keys. The action is multiplied by the influence. If you the motion to be small, use an influence close to 0, if you want the full animation amplitude, use and influence of 1. With this technique, one action would be enough to slowly increase the strength of movement over time.


The animation results from first increasing the influence of the windAction, then blending over to the strongerWindAction.